Ancient Tsunamis Reveal New Zealand’s Earthquake Risk

Scientists have uncovered evidence of ancient tsunamis at Lake Grassmere on New Zealand’s South Island, providing the first reliable estimate of how often massive earthquakes strike the region’s major fault system.

The Discovery
Researchers drilled deep into the lake bed and found unusual layers packed with sea shells and sand — clear signs that powerful tsunami waves had swept inland from the coast. These deposits revealed two ancient tsunamis that occurred approximately 2,000 and 1,400 years ago. The shells were the key evidence. Finding dense concentrations of marine life in what should be a freshwater lake environment could only mean one thing: massive waves had carried them there during devastating tsunami events.

What the Pattern Reveals
Combined with previous research from nearby sites, scientists now have a record of four major earthquakes over the past 2,000 years along the Hikurangi Subduction Zone — New Zealand's largest fault system beneath the ocean floor. The analysis reveals these catastrophic earthquakes occur roughly every 500 years with surprising regularity. Since the most recent major earthquake happened about 500 years ago, the fault system may be approaching its next active phase.

Current Risk
Based on this historical pattern, researchers calculate a 26% probability of a major subduction earthquake occurring within the next 50 years. This poses significant risks for New Zealand, particularly around Wellington and the Cook Strait region, where such an earthquake would likely trigger widespread tsunamis. The research provides crucial data for emergency planning and demonstrates how scientists can read the landscape’s ancient history to better understand future risks.